The present invention relates to a ball screw used to threadedly engage a shaft and a nut with each other through a large number of balls and, in particular, to a ball screw which can cope with a large rating load and a reduction in noises.
Generally, in order to increase the rigidity and rating load of a ball screw, the ball screw is designed to increase number of balls receiving a load within a nut of the ball screw.
However, in case where the ball screw is designed such that the number of balls in a ball line defined by a single circulation member (a member which allows the balls to circulate endlessly therealong) is increased, the balls are easy to rub against each other in the area where the loads applied to the balls, which has an ill effect on the operation performance of the ball screw. It is also known that such mutual rubbing actions of the balls have an ill effect not only on the operation performance of the ball screw but also on the wear of the balls and ball grooves.
To avoid the above drawbacks, conventionally, there is employed a system in which the closed circuit of the ball line defined by the single circulation member is divided into a plurality of circuits, that is, as shown in FIG. 6, a plurality of ball 2 lines are respectively defined by a plurality of circulation members 6. In FIG. 6, reference character 1 designates a shaft, 2 a ball, and 3 a nut, respectively.
However, in the above-mentioned conventional ball screw, in case where the number of circulation members 6 is increased, there is increased the number of portions where noises are generated, which increases the scale of the noises as a whole. Also, as shown in FIG. 6, the number of times of which the line of balls 2 held by one circulation member 6 is wound around the periphery of the shaft 1, that is, the number of windings of the ball 2 line cannot help being the numbers of windings at intervals of 0.5, for example, 2.5 windings and 3.5 windings. Due to this, in FIG. 6, referring to the respective total numbers of balls 2 in the upper and lower half sections of the shaft 1 when the shaft 1 is viewed from the side surface side thereof, in the upper half section, the ball 2 line is wound 3.0 times and, on the other hand, in the lower half section, the ball 2 line is wound 4.5 times. Thus, there is found a difference of 1.5 windings between the upper and lower half sections. This shows that there is a difference between the upper and lower half sections in the number of balls 2 introduced.
Therefore, in case where the thus structured ball screw receives an external load, the loads applied to the respective balls 2 do not balance well with each other, which can cause the balls 2 to flaking and can shorten the anti-abrasion lives of the balls 2. Also, since there is formed a portion into which the balls 2 are not introduced between the closed circuits of the balls 2 lines the length of the nut 3 cannot be used effectively. In addition, such increase in the number of circulation members 6 also gives rise to an increase in the cost of the ball screw.
In view of the above, conventionally, there is proposed another ball screw which, as shown in FIG. 7, is improved in the balance of the loads to be applied to the balls 2. In the ball screw shown in FIG. 7, of the three circulation members 6 shown in FIG. 6, the centrally-situated circulation member 6 is mounted at a position reversed by 180°. In FIG. 7, referring to the respective total numbers of balls 2 in the upper and lower half sections of the shaft 1 when the shaft 1 is viewed from the side surface side thereof, the line of the balls 2 is wound 3.5 times in the upper half section, whereas the balls 2 line is wound 4.0 times in the lower half section, thereby providing a difference of 0.5 windings between the upper and lower half sections. That is, the difference is reduced by 1.0 windings when compared with the conventional ball screw shown in FIG. 6. Thanks to this, the balance between the upper and lower half sections of the balls 2 line is improved, which makes it possible to prolong the life of the ball screw.
However, since the number of circulation members 6 is equal to that of the ball screw shown in FIG. 6 and there increases the number of portions between the closed circuits of the ball 2 lines into which the balls 2 are not introduced, in case where the number of balls 2 to receive the load is set equal to that of the ball screw shown in FIG. 6, the length of the nut 3 is enlarged.